Aviation gasoline



'from United States Patent This. invention relates to improved fuels for internal combustion engines.

In one of its morespecific aspects,

it relates to'an aviation fuel conforming. to rigid specifications ofanti-knock rating, vapor pressure, and distillation characteristics, and having in addition, characteristics whichresult in providing substantially even distribution of highanti-knock portions of the finished fuel .to. all of the cylinders of a multicylinder, aviation-type, spark ignition, internal combustion engine. Inanother specific aspect, i'trelates to a improved operation of an aviation engine. i

This is a continuation of our U. S..application, Serial No. 750,174, filed May 23, 1947, and now abandoned.

Among other requisites which must be met in a superior aviation gasoline blend is that of high anti-knock properties. This requirement resulted from the discovery that engine knock is a function of fuel composition as well as engine conditions.

Many approaches! have been? made to the problem of raising anti-knock characteristics of fuels. One of the first such approaches was. made with the use of additives or anti-knock tetraethyl lead, aniline, carbonyls of ironor nickel, etc., whose sole purpose, when added to gasoline was-to act as knock inhibitors and increase the anti-knock rating.

agents, e. g.

ICC

to all cylinders, is commonrknowledge to theaircraft and petroleum industries. Those-cylinders which receive the leanest fuel-air ratio have an increased tendency'to detonate. The more volatile fuel fraction, in most en gines,.travels-to the cylinders farthest from the carburetion point while the heavier fractions are burned in the nearest cylinders. Since those cylinders which receive the leanest mixture of gasoline and air tend to receive the more volatile constituents of the original gasoline charge, i. e., those constituents which are vaporized first under given vconditions, we have discovered that it is very desirable that these more volatile constitutents be as high in anti-detonation characteristics as possible to permit operation in all cylinders at the highest possible knock limited horsepower under all engine conditions.

An object of our invention'isto provide fuel of high anti-knock quality.

A still further object of our invention is to provide an aviation fuel which will result in improved engine performance, insofar as maximum obtainable knock limited horsepower is concerned, under all operating conditions.

A. still further object of our invention is to provide an improvedmethod for operating an aviation engine.

Another object of our invention is to provide a fuel which will give better comparable performance inall comparable engines, regardless of the engine fuel distribution characteristics, than can be obtained with ordinary aviation gasolines.

A further object is to provide a means and method of more effectively utilizing the B. t. u. potential of an aviation gasoline.

Other objects of the invention will be apparent from v the discussion and description contained herein.

Thereafter emphasis was placed on blending of specific fuels according to rigid specifications and' in conjunction 4 It has generally been believed heretofore that'eflici'ency with the use of knock inhibitors.

ofanaviation fuel can be pretty well predicted-from the ASTM. octane number. We have, however, ,nowdiscovered that such is not always the case. Two fuels having essentially the same octane numbermay, when usedin: actual flight, vary quite radically in actual'performance. The knock limited horsepower derived from;

one fuel may be substantiallygreater.v than that gained from another fuel having the same octane number and the same anti-knock rating, when usedin .the..samel en-' gine. Aircraft engines, which have-been designedto operateon fuels falling within a specific octanenumber range, can-now be operated by meansof our invention on: fuels which fall'substantia-lly below the design octane range of theengine;

Knock limited" horsepower, as used. hereiniswth'e maximum horsepower output obtainable witlr'a given fuel inianfaviation-type, spark ignition,.,internal.combustion engine underspecific operating conditions 'that will not re'sult'in knocking. This is tolbe. distinguished knocklimitedhorsepower as used' in the'auto motive industry, wherein it means the horsepower reached at'a point where power. falls ofl? due to knocking: O.ur invention" provides anaviation fuel whichyields. an in creased knock limited horsepower, under conditions of ing the laboratory octane rating of the fuel as measured by-ASTM-AviationMethod'Designation'D614-44T;

The fact that multi'cylinder internal combustion aircraft sustained. flight,.over. that provided byordinary fuels". This improved performance is obtained without increase engines do. not distribute the charge of-gasolineequally Althoughdiiferent aviation gasolines may have substantially identical laboratory anti-knock ratings, no at tempt has been made, heretofore, to substantially concentrate thehighestquality anti-knock components of an aviation fuel in the low boiling blend portion where they will allow the engineto give the greatest overall knoek limited horsepower and make the best utilization of the B. t. u. potential of the fuel. As pointed out above, we have'discovered'that'by concentrating high anti-knock properties in substantially the lowest boiling 45-55 volume per. cent fuel blend we increase the knock limitedhorse= power which will be produced by an aviation engine using such fuel under lean fuel-air ratio conditions. By thus increasingthe knock limited horsepower wemore efliciently make u'se 'ofthe- B. t. u. potential of thefueli Fuelshavinghigh'anti-knock components in the higher boiling. portions require even distribution of the unvaporized fractions, if any exist, under all operating condi'tions" to givemaximum anti-knock performance; This type'fuel depreciates in flight anti-knock value as con ditions developthat result in uneven distribution of the unvaporized fractions, while the fuelsof our invention,

having high anti-knock components in the low boiling fraction, give comparatively better flight knock limited horsepower performance under such conditions.

The special aviation fuel of our invention has shown a superior performance on actual flight tests because it compensates for the lack of perfect fuel distribution in the engine. It is known that the lean mixture cylinders receive a disproportionately high proportion of the lighter fractions of the fuel and limit the knock free performance oftheengine: Thusit willbeseen that those cylinders receiving. a lean mixture require a higher anti-knock fuel, toprovide knock free performance, than do those cylinderswhi'ch"receive therichestmixtures. We have also found-that specialk'noek inhibitors or anti-knock agents,-

in an engine having imperfect fuel distribution characteristics. By the practice of our invention one tends to correct the premature knock limit by blending an aviation gasoline so that the lighter fractions comprise the highest octane quality ingredients and thus the knock free furnish a substantial portion of the power which was formerly furnished by the defective engine. It will thus be noted that a considerable safety factor is necessary in a fuel or engine in order to furnish the additional power when required. Aircraft engines as now conpower output the epgme Increased structcd and designed fall within the fuel requirement T present Invention may be employed m a great class s of 91 or 100 minimum octane number or 115 variety of blending operations involving all suitable blendf b d ing agents and base stocks. One such procedure is outmlmmum per.ormance i an .englries are lined in the following Operations A fuel may be pref contemplated in fuel requlrement classification considerpared according to a blending formula which requires in f f f l f Performance number Such d fi i proportions f normal hexane, ismhexana g in the vicinity of 130 mimmum performance number and heavy alkylate and a heavy naphtha comprising hydroabove- Octane numbfif y be defined as the P cent carbons containing 1.1 to 12 carbon atoms per molecule y volume 05 filo-Octane tlimethylpentafle) that in order to produce a composition having an octane nummust be mixed with normal heptane in order to match her between 83 and 85, determined by the ASTM Avia- ID the knock intensity of the unknown fuel and may be non Mfithod Deslgna'ilon with a Elven determined by ASTM Aviation Method Designation centration of tetraethyl lead. Said blending formula is 514. 44 P f a number i an average value devised so that the composite fuel Wlll have a Reid indicating the per cent of knock limited power output f l i of f l f hp developed with a fuel based on the knock limited power 1 "t 0 e 1mpr0.ve i ormance c l f output of iso-octane (2-2-4, trimethylpentane) as 100. teristics obtained by use of a high anti-knock low boiling Performance numbers bear a relation to ratings of ISO- fractron to furnish overall high anti-knock performance t 1 TEL d S u d t ed f under lean mixture conditions is shown in the following 00 P p us an an: y e ermm tom con data version charts or tables of iso-octane plus TEL. vs. per- Example formance numbers. For the purpose of simplicity we d b will use hereinafter the term anti-knock rating rather than An avlaflon fuel blend (1) was prepare y imxmg octane number or performance number but which term two fuels Shown as formulas A and B together wlt.h will include both It will be understood that when the proximately 4 ml. tetraethyl lead per gallon. Avratlon imbkuod, ratin 100 or below We mean octane fuel blend (2) was prepared by mixing two fuels shown d 2 d d h th a {401 k rafin is as formulas C and D together with approximately 4 ml. er as a (We e ne an W en e g tetraethyl lead per gallon.

Composite, volume percent AS-TM Boiling Reid ml. TEL Aviatlon Blend Range, vapor per Method Formula F. pressure HF Heavy Gallon Designan-hexane i-hexane Heavy Naphtha tron Alkylate D614-44'1 While it will be noted that blend (2) has a slightly better ASTM Aviation Method Octane Number than blend (1), the higher boiling fraction of blend (2) has a concentration of higher anti-knock properties, and in blend (1) the lower boiling fraction has the concentration of higher anti-knock properties. In each case all components are saturated and primarily parafiinic. Blends (1) and (2) were tested in actual flight using a C49K airplane equipped with Wright R-1820-87 engines. The following data on knock limited horsepower were obtained while operating the engines at 1750 R. P. M. and at 33 F. carburetor air temperature.

Knock limited horsepower Knock Value 1 Performance Number Performance Number Knock Value 1 Fuel-Air Ratio .060 .065 .070 .075 .080

d 1 540 570 594 612 621 ig d i2; 531 560 584 606 620 sary for the remaining operable engineor engines to 1 Knock value: ml. TEL/U S gal. in iso-octane with such knock indicating instruments, give from 2 to 25' per cent more knock limited horsepower than will ordinary fuels under'lean mixture conditions. A lean mixture is considered by'many within the motor fuel and aviation industries to be a fuel-air ratio smaller than 0.09 but not lower than .05. Those greaterthan0.09 are considered to be'rich mixtures. A fuel which gives the necessary power and high knock limited performance under lean mixture conditions is highly desirable as the main fuel due to the increased economy of operation obtained.

It is common knowledge that aircraft engines, as classified in the 91, 100, 115 and higher anti-knock classifications, have safety factors and will ordinarily operate commercially on ordinaryfuels with anti-knock ratings below the minimum anti-knock rating of the design classifications but still within the safety factors of said engines. It would be extremely. difiicult to say that any one engine has a specific safety factor of a certain span of anti-knock units. Infact that would-not be true for, under ideal flying conditions, an engine would operate satisfactorily on ordinary fuels having an anti-knock rating considerably below the minimum anti-knock rating classification of the engine. Under moreseverefllying conditions it willbe noted that the safety factor has been caused to shrink and will take .a higher anti-knock rated fuel to give absolutely knock free performance under commercial flying conditions. By operating the sameengine on a fuel blended according to our'invention we in effect increase the safety'factor of the engine. A fuel blended according to our invention will commercially operate that engine and giveknock free performance when the fuel has an anti-knock rating between 2 and anti-knock units below that of an ordinary fuel which will operate the engine at the lower limit of its fluctuating safety factor. Of course as a matter of common practice engines are not actually operated commercially with fuels which will barely give knock free performance within the lower margin of the safety factor. The purpose of this discussion is, however, to point out the fact that additional safety as well as power is derived in operation with a fuel blended according to this invention.

The results of the preceding test indicate that a fuel having high anti-knock components substantially concentrated in a low boiling blend comprising in the neighborhood of 50 such as 45 to 55 volume per cent of the composite blend will give substantially greater horsepower in actual flight under lean fuel conditions than will a fuel of nearly the same anti-knock rating but having the high anti-knock components substantially concentrated in the high boiling fraction.

One other point of interest is the fact that many engines have even smaller safety factors under their class rating than the specific test engines used in this test. The difference in knock limited horsepower developed with the two fuels in use with these test engines is indicative of the greated difference in results which are obtained with engines having carburetion distribution characteristics comparably less efficient than these test engines. The Wright R-182O engine, as is well known in the aviation industry, is among the most efiicient of aviation gasoline engines in the respect of carburetion distribution characteristics. With an engine having poorer carburetion distribution characteristics, the results from the use of a fuel of our invention are even more outstanding.

"Whilethe foregoing specific example providesan illustration of the improvement obtained through practice of theipresent' invention, it will be apparent that the variations which 'canbe produced by changing the volumeratio, quality 'and hydrocarbon type of the blending components, are numerous. t

The blending stock or fuel blend'which is to comprise the lightend ofthe' finishedfuel shouldfall'within a distillation range, as determined by ASTM Method Designation D-86 of from about 75 to F.'to about 225 F., have an anti-knock rating ofbetween land 20 antiknock units more than that desired in the finished blend when' containing an amount of'anti=knock'agent equal'or equivalent totthe'amount contained in the finished fuel. 'lheblending-fuel comprising the heavy end of the finished fuel should'fall within .ajdistillation range, as :determinedby ASTM MethodDesignation D-86of from about 175 F. to about 410" IF., have'an anti-knock ratingof'between 1 and 20 anti-knock units less thanithat desired in the finishedfuel when containing an amount of anti-knock agent the same as the amount contained in the "finished fuel, which if containing tetraethyl lead fluid should contain between 0 and- 6.ml. per gallon. As the anti-knock rating requirement is increased the variance of the lightand heavy blend anti-knock ratings from that desired in the finishedblend will diminishproportionally'from' 7 to 20 a'nti-knockunits for a 91 minimum anti-knock rating engine to'l to 6 anti-knock units for a 135 minimum anti-knock rating classification engine. The variance of the light and heavy blend antiknock ratings from that desiredin the finished blendfmay fall within a difference of from 1" to 12 anti-knock units fora or aboveminimum class engine. The finished 'fuel of such. a blend should have'j'distillation characteristics of gasoline as "determined? by ASTM Method Designation-D-86 and'have' a Reid vapor pressure of from about 2to'9 pounds with afpressurebetween 4 and'7 pounds being preferred.

We have disclosed in our specific'example a. finished aviation fuel with an anti-knock'ratingdn the vicinity'of 80'which is nearly at'theb'ottorn of the anti knock 'ra'ting range desirable for aviation gasolines. In preparation of higher anti-knock rating gasoline it will be necessary to remove substantially all normal parafiins from the light end blending fuel and use mixtures of isomeric hexanes comprising diisopropyl and neohexane, and mixtures of naphthenic hydrocarbons or naphthenic concentrates which may comprise cyclopentane, methylcyclopentane, methylcyclohexane and cyclohexane. In the higher boiling end aliphatic paratfins, cycloparafiins and nephthenes may be used with aromatics. As the antiknock rating requirement increases normal-parafiins will be more completely decreased.

We claim:

1. A hydrocarbon fuel composition consisting essentially of a first hydrocarbon fraction comprising 45 to 55 per cent by volume of the finished fuel and consisting essentially of about 25 volume per cent of normal hexane and about 75 volume per cent of iso-hexane; a second hydrocarbon fraction comprising 45 to 55 per cent by volume of the finished fuel and consisting essentially of about 55 volume per cent of HF heavy alkylate boiling within the range of 357 F. to 391 F. and about 45 volume per cent heavy naphtha comprising essentially hydrocarbons containing 11 to 12 carbon atoms per molecule; and about 4 ml. tetraethyl lead, said fuel having distillation range characteristics of gasoline as determined by ASTM Motor Designation D-86, and a Reid vapor pressure of about 4 pounds.

2. A hydrocarbon fuel composition consisting essentially of a first hydrocarbon fraction comprising 45 to 55 per cent by volume of the finished fuel consisting essentially of hexanes, at least 75 volume per cent being at least one iso-hexane; a second hydrocarbon fraction comprising 45 to 55 per cent by volume of the finished fuel and consisting essentially of HF heavy. alkylate boiling within the range of 357 F. to 391 F. and heavy naphtha comprising essentially hydrocarbons containing 1.1 to 12 carbon atoms per molecule, said second fraction having a lower anti-knock rating than said first fraction; and up to 6 ml. tetraethyl lead per gallon, said fuel having distillation range characteristics of gasoline as determined by ASTM Motor Designation D-86 and a Reid vapor pressure of between 2 and 9 pounds.

3. A hydrocarbon fuel composition consisting essentially of a first hydrocarbon fraction comprising 45 to 55 per cent by volume of the finished fuel consisting essentially of hexanes, at least 75 volume per cent being at least one iso-hexane; a second hydrocarbon fraction comprising 4-5 to 55 per cent by volume of the finished fuel and consisting essentially of about 55 volume per cent of HF heavy alkylate boiling Within the range of 357 F. to 391 F. and about 45 volume per cent heavy naphtha comprising essentially hydrocarbons containing 11 to 12 carbon atoms per molecule; and up to 6 ml. tetraethyl lead per gallon, said fuel having distillation range characteristics of gasoline as determined by ASTM Motor Designation D-86, and a Reid vapor pressure of between 2 and 9 pounds.

4. The fuel composition of claim 3, wherein said isohexane comprises diisopropyl.

5. The fuel composition of claim 3, wherein said isohexane comprises neohexane.

6. The fuel composition of claim 3, wherein said isohexane comprises a mixture of diisopropyl and neohexane.

7. The fuel composition of claim 3, wherein the fuel has a Reid vapor pressure of between 4 and 7 pounds.

8. A hydrocarbon fuel composition consisting essentially of a first hydrocarbon fraction comprising 45 to 55 per cent by volume of the finished fuel consisting essentially of C6 parafiins, at least 75 volume per cent being at least one Cs iso-paraflin; a second hydrocarbon fraction comprising 45 to 55 per cent by volume of the finished fuel and consisting of a mixture of hydrocarbons selected from the group consisting of parafi'ins, cycloparaflius, naphthenes and aromatics boiling Within the range of 175 F. to 410 F., said second fraction having a lower anti-knock rating than said first fraction; and up to 6 ml. tetraethyl lead per gallon, said fuel having distillation range characteristics of gasoline as determined by ASTM Motor Designation D-86 and a Reid vapor pressure of between 2 and 9 pounds.

9. A hydrocarbon fuel composition consisting essentially of a first hydrocarbon fraction comprising to per cent by volume of the finished fuel consisting essentiaily of C6 paraflins, C5 and Cs cycloparaflins and at least volume per cent being at least one hydrocarbon selected from the group consisting of iso-paraffins and cycloparafiins; a second hydrocarbon fraction comprising 45 to 55 per cent by volume of the finished fuel and consisting of a mixture of hydrocarbons selected from the group consisting of parafiins, cycloparaffins, naphthenes and aromatics boiling within the range of F. to 410 B, said second fraction having a lower anti-knock rating than said first fraction; and up to 6 ml. tetraethyl lead per gallon, said fuel having distillation range characteristics of gasoline as determined by ASTM Motor Designation D-86 and a Reid vapor pressure of between 2 and 9 pounds.

10. The hydrocarbon fuel of claim 2, wherein said HF heavy alkylate is at least 55 per cent of said second hydrocarbon fraction.

References Cited in the file of this patent UNITED STATES PATENTS 2,407,716 Marschner Sept. 17, 1946 2,409,156 Schulze Oct. 8, 1946 2,445,360 Miller July 20, 1948 OTHER REFERENCES Aviation Gasoline Manufacture-Van WinkleMc- Graw-Hill Book Co., Inc., New York, N. Y., 1944, pp. 47, 70, 198, 199 and 241.

Aviation Gasoline ManufacturerVan Winkle-Mc- Graw-Hill Book Co., New York, N. Y., 1944, pp. 107-8, 204, 210-211. 

1. A HYDROCARBON FUEL COMPOSITION CONSISITNG ESSENTIALLY OF A FIRST HYDROCARBON FRACTION COMPRISING 45 TO 55 PER CENT BY VOLUME OF THE FINISHED FUEL AND CONSISTING ESSENTIALLY OF ABOUT 25 VOLUME PERCENT OF NORMAL HEXANE AND ABOUT 75 VOLUME PER CENT OF ISO-HEXANE; A SECOND HYDROCARBON FRACTION COMPRISING 45 TO 55 PER CENT BY VOLUME OF THE FINISHED FUEL AND CONSISTING ESSENTIALLY OF ABOUT 55 VOLUME PER CENT OF HF HEAVY ALKYLATE BOILING WITHIN THE RANGE OF 357* F. TO 391* F. AND ABOUT 45 VOLUME PER CENT HEAVY NAPHTHA COMPRISING ESSENTIALLY HYDROCARBONS CONTAINING 11 TO 12 CARBON ATOMS PER MOLECULE; AND ABOUT 4 ML. TETRAETHYL LEAD, SAID FUEL HAVING DISTILLATION RANGE CHARACTERISTICS OF GASOLINE AS DETERMINED BY ASTM MOTOR DESIGNATION D-86, AND A REID VAPOR PRESSURE OF ABOUT 4 POUNDS. 